1. Field of the Invention
The present invention relates to power management for battery powered devices, and more particularly, to a power management topology that includes an external AC/DC adapter that is controlled by battery charge controller associated with a portable device.
2. Background Description
Present battery charging topologies are divided into two separate designs and implementation: AC adapters and battery charging topologies.
AC adapters have two present designs:
1) 60 Hz—Low cost transformers with full wave rectifiers and a filter capacitor. The windings of the transformer usually have high resistance that results in a quasi constant current source.
2) Hi Frequency—Promoted as the travel version of AC adapters with high frequency usually>100 KHz. As reactive impedance is a direct function of frequency (Xl=2 πf L and Xc=½ πf C) for the same impedance L and C are smaller by the ratio of frequencies. For example the size of an inductor for 600,000 Hz vs 60 Hz is the ratio of 60/600,000 or 1/10,000=0.0001. These travel versions of AC adapters are designed with high frequency Switch Mode Power Supplies (SMPS). Hence the benefits are small size and light weight, highly valued by travelers, but may cost more than other types of adapters.
Most adapters in use today include PWM circuitry and controllers (including power switches and DC/DC converter circuitry such as Buck, flyback, boost, bridge, or other type of converter topology) to generate a regulated output.
Battery Chargers in systems like notebook computers, cellular phones and PDA's are generally used to control battery charging and/or power distribution to a system. Battery chargers generally have three popular designs:
1) Simple switched adapter charger uses a single electronic switch to directly connect the adapter to the battery. Then turning the switch off when the final charge voltage is reached. While relatively inexpensive, this type of charger circuitry must use a constant current AC Adapter generally the heavy, 60 Hz type. The battery charging algorithm is highly compromised, resulting in long charge times, perhaps never reaching full charge and limited ability to adapt to multiple battery chemistries like LiIon, NiMH and NiCd.
2) Linear regulators—Creation of a fixed output voltage is developed by dissipating excess input voltage within the regulating component. This usually results in efficiencies of 50% or less. The wasted power is dissipated within the regulator increasing the temperature within the small, tightly enclosed product. Additionally the wasted power significantly shortens the battery life which is of paramount importance to anyone carrying these products around. The only benefit for a consumer product with a dead battery is as an expensive piece of exercise equipment. The benefits of linear regulators are simplicity and low cost. The negatives include short battery life and high internal temperatures.
3) Switchmode Regulators—As described above, this method uses a switched mode power supply to efficiently (90 to 95%) convert the input voltage to the battery charge voltage. Optimum charging algorithms can be applied like constant current mode switching to constant voltage mode. Benefits of this type of design are rapid charging, high efficiency and adaptability to varying adapters and battery chemistries, but may cost more than linear regulators.
FIG. 1 depicts a conventional power management topology for a portable device. The system includes a portable device 10 that includes one or more batteries 30 and one or more active systems 18, 20, and/or 22 coupled to an AC/DC adapter 12. The adapter 12 operates to deliver controlled power to both charge the batteries and power any systems coupled thereto. A battery charger circuit 14 is provided to provide regulated power (voltage and/or current) to the battery 30 based on, for example, battery charging current, battery voltage, and/or available power from the adapter 12. Referring to FIG. 1A, a block diagram of a conventional battery charger circuit 14 is depicted. As is well understood in the art, the charger generally includes a plurality of error amplifiers 34 that monitor battery voltage and/or current and generate an error signal if the battery voltage and/or current exceed some predetermined threshold. Additionally, an error amplifier may be included to monitor input power availability and generate an error signal if the available power from the adapter 12 is exceeded. The charger 14 also includes a PWM generator and controller 36. The error signals generated by the error amplifiers are received by the controller 36 and operate to adjust the duty cycle of the PWM generator. The PWM signal is supplied to power switches and DC/DC converter 38 to generate a regulated DC source for charging the batteries.
Similarly, the AC/DC adapter 12 includes a PWM generator and controller, and further includes power switches and a DC/DC converter to provide a regulated output power source. Thus, a redundancy exists since both the adapter 12 and the charger 14 include a PWM generator and controller, power switches and a DC/DC converter.